Purity: Validity of Early Ghusl with Obligatory Intention
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 358:
If a person is certain that the time for prayers has set in and he makes the intention of performing an obligatory ghusl, and afterwards he realises that actually he performed ghusl before the time for prayers had set in, his ghusl is valid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)